The initial proposal for scalable optical quantum computing required single photon sources, linear optical elements such as beamsplitters and phaseshifters, and photon detection. Here we demonstrate a two qubit gate using indistinguishable photons from a quantum dot in a pillar microcavity. As the emitter, the optical circuitry, and the detectors are all semiconductor, this is a promising approach towards creating a fully integrated device for scalable quantum computing.

We have demonstrated logical operation of an all semiconductor optical CNOT gate operating with indistinguishable single photons. These results are promising for the scalability of future photonic quantum computing technology. Extending our scheme to incorporate multiple single photon sources will allow deterministic preparation of multiphoton input states. Integration of the photon source and detectors directly into photonic circuitry will allow further miniaturization and reduction of the resources required.

Other labs have created photon circuits in the form of interconnecting waveguides that force photons to interact and thereby process the information they carry. These circuits are like tiny Scalectrix sets in which the cars collide where the track narrows.

In this circuit, the qubits are path-encoded meaning that the presence of a photon in one track is a 1 and its absence is a 0, for example. When they come together they interfere, thereby processing the information they carry.

But the efficiency of this interaction depends crucially on both photons being identical. Small differences in wavelength, for example, can dramatically reduce the performance. But making identical photons is hard.

Andrew Shields at Toshiba Research Europe Limited in Cambridge, UK, and a few buddies say they’ve solved this problem by integrating both these developments into a single device that acts like a C-NOT logic gate.

“The Controlled-NOT (CNOT) gate we demonstrate is the basic building block of quantum logic, since in combination with one qubit gates it can be used to perform any quantum operation,” say Shields and co.

This logic gate consists of a pillar of indium arsenic that acts as a quantum dot–it emits a single photon when zapped with laser light of a specific frequency. This is coupled to a photon racetrack carved out of silicon.

A C-NOT logic gate requires two input photons. So the circuit works by zapping the quantum dot twice, generating two photons. These are identical because they’ve come from the same dot.

The photons then travel to a beam splitter that sends them down the appropriate paths (one of which has a built in delay that means determines when the photons enter the circuit relative to each other).

Shields and co have measured the truth table of their logic gate. They say it matches their theoretical predictions and can be made better with a few tweaks.

What’s significant about this approach is its scalability. Shields and co say it ought to be possible to build many quantum dots and circuits onto a single integrated chip. And the differences between photons from different quantum dots can be minimised by triggering them all with the same laser pulse.

That’s handy but it’s not all plain sailing. The device must be cooled to 4.5 Kelvin, the operating temperature of the quantum dot, and the results of a single logic operation take some 30 minutes to collect.